1. Field of the Invention
The invention relates generally to generating ultrashort pulses of laser light. The invention relates particularly to the use of specialized gain media in the laser system, in order to generate laser pulses of minimal temporal width. The invention relates most specifically to the use of neodymium-doped phosphate laser glass that offers maximally broadened emission bandwidths, so as to allow for the generation of minimally short pulses of laser light.
2. Description of Related Art
It was first recognized that neodymium-doped (Nd) glass could serve as a laser material several decades ago (E. Snitzer, "Optical Maser Action in Barium Crown Glass," Physical Review Letters 7, 444 (1961)). The Nd:glass must be energized with a pump source, such as a flashlamp, laser diode, or other laser, so that the material exhibits gain near 1054 nm. In this way it is possible to amplify light input to the gain medium, or to generate a laser beam by situating the gain medium in an oscillator. Following the recognition that Nd:glass was a useful gain medium, the modelocked Nd:glass oscillator was reported. (A. J. DeMaria, D. A. Stetser, and H. Heynau, "Self-Mode-Locking of Laser with Saturable Absorber," Applied Physics Letters 8, 174 (1966)). Here, a train of about a hundred pulses is obtained, such that each individual pulse is about 10 psec long. This concept of modelocking flashlamp-pumped Nd:glass lasers with a saturable absorber proved to be of great interest to the scientific community, (for example, see M. A. Duguay, J. Hansen, and S. L. Shapiro, "Study of the Nd:glass Laser Radiation," IEEE Journal of Quantum Electronics 6, 725 (1970); A. Laubereau and W. Kaiser, "Generation and Applications of Passively Mode-Locked Picosecond Light Pulses," Opto-Electronics 6, 1 (1974)). While this early method was both simple and inexpensive in that only a saturable absorber dye was needed, the laser output tended to be unstable and the use of the dye was problematic in some circumstances. Many alternative methods of modelocking solid state lasers were examined, although the so-called self-modelocking technique proved to be of the greatest interest, in part because it was possible to generate pulses with sub-picosecond duration (for example, see W. Sibbett, R. S. Grant, and D. E. Spence, "Broadly Tunable Femtosecond Solid State Laser Sources," Applied Physics B 58, 171 (1994)). The specific adaptation of generating femtosecond-duration pulses with Nd:glass was pursued by Keller et al. (U. Keller, T. H. Chinn, and J. F. Ferguson, "Self-Starting Femtosecond Mode-Locked Nd:glass Laser that Uses Intracavity Saturable Absorbers," Optics Letters 18, 1077 (1993)), who demonstrated the generation of 130 fsec pulses from a laser-pumped Nd:glass system. Other workers have recognized the value of employing laser diodes as the pump source for a modelocked Nd:glass system (U.S. Pat. No. 4,951,294, Basu et al.). The prior art appears to be concerned with the preferred means of modelocking Nd-doped glasses and other gain media by way of improved techniques and cavity arrangements, while the present invention relates to the preferred types of Nd:glass that will minimize the output pulsewidth of the laser oscillator.
The pulse duration and spectral width of the output from a modelocked laser are fundamentally constrained by the relationship: EQU .DELTA..nu..multidot..DELTA..tau.&gt;0.3
where .DELTA..nu. is the full-width-at-half-maximum (FWHM) of spectral bandwidth in s.sup.-1 and .DELTA..tau. is the temporal FWHM in seconds. As a consequence of this relationship, gain media that offer a broader emission spectrum can generally be configured to generate shorter pulses in a modelocked oscillator. On this basis alone, Nd-doped silicate glasses would be preferred over phosphate glasses because they have greater emission bandwidth. Silicate glasses, however, are not commonly employed today because it is not possible to melt these types of materials such that they are free of platinum inclusions using existing manufacturing methods. The platinum inclusions tend to have a very low optical damage threshold, rendering the silicate glasses to be somewhat less desirable for use in lasers, while it turns out that it is generally feasible to completely eliminate the inclusions from phosphate-based glasses. It is for these reasons that a Nd-doped phosphate glass that offers maximal spectral width would produce the advantage of the generation of shorter output pulses in a modelocked oscillator.
The first Nd-doped phosphate laser glass patents focused solely on the composition of the material (U.S. Pat. No. 3,250,721, DePaolis et al.), although subsequent patents tended to become more specialized so as to meet the objectives of certain types of lasers. For example, Deutschbein et al., disclose phosphate laser glass compositions having small expansion coefficients and negative dn/dT values (i.e., the change in refractive index versus temperature) in order to devise athermal laser glasses that offer reduced thermal lensing (U.S. Pat. No. 4,022,707). U.S. Pat. No. 3,979,322 by Alexeev et al., also acknowledges the significance of dn/dT as well as the stimulated emission cross section .sigma..sub.em, and claims various phosphate glass compositions suitable for Nd-lasers. Other patents disclose the appropriate compositions that allow for reduced glass transition temperature (U.S. Pat. No. 4,996,172 by Beall et al.); improved thermal shock resistance and suitable laser-optical properties (U.S. Pat. No. 4,820,662 by Izumitani et al., U.S. Pat. No. 4,929,387, by Hayden et al., U.S. Pat. No. 5,053,165 by Toratani et al.,); water durability devitrification tendencies as well as favorable laser-optical properties (U.S. Pat. No. 4,075,120 by Myers et al.,); chemical strengthening techniques (U.S. Pat. No. 5,164,343 by Myers); athermal behavior (U.S. Pat. No. 4,333,848 by Myers et al.); and concentration quenching (U.S. Pat. No. 4,371,965 by Lempicki et al. and 4,470,922 by Denker et al.). The aforementioned patents are cases where glass compositions were tailored to offer properties favorable for laser operation. In none of these cases was the emission bandwidth called out as a significant quality with respect to laser performance.
There are, however, several patents that are more closely related to the present invention. For example, the composition of the present glass is explicitly noted in U.S. Pat. No. 5,032,315 by Hayden et al. The current invention, however, relates to the use of this particular type of Nd:glass in a laser system designed to generate or amplify ultrashort pulses of light, where the gain medium was selected to provide a particularly broad emission bandwidth. The relationship between laser glass properties and the capability of generating ultrashort pulses was considered in U.S. Pat. No. 4,239,645 by Izumitani et al., although it was solely considered on the basis of the nonlinear refractive index, n.sub.2, of the material. The importance of the Nd emission bandwidth .DELTA..lambda..sub.em was explicitly mentioned in two patents. In U.S. Pat. No. 4,661,284 by Cook et al., .DELTA..lambda..sub.em was specifically recognized although the intent was to identify glasses where its magnitude was minimized. In U.S. Pat. No. 5,173,456, a preferred embodiment was that .DELTA..lambda..sub.em be in the range of 27.0-30.5 nm. The utility of the glass was described to be that of providing enhanced bandwidth of the laser output in order to reduce the coherence of the beam. The present invention is intended to relate to the use of certain preferred types of laser glasses in an ultrashort pulse laser.
In view of the keen interest in devising lasers that can deliver ever shorter pulses, in concert with the capability of tailoring the properties of phosphate glasses to meet certain requirements, the object of the current invention is to link these two situations in order to devise an improved means of generating the shortest possible laser pulses.